Magnetic disk and manufacturing method thereof

ABSTRACT

In a magnetic disk having a magnetic layer, a protection layer, and a lubrication layer formed on a substrate in this order, a surface free energy γS of a surface of the magnetic disk derived by an extended Fowkes equation is greater than 0 and no greater than 24 mN/m. γSd (dispersion force component of surface free energy) forming the surface free energy γS is greater than 0 and no greater than 17 mN/m, γSp (dipole component of surface free energy) forming the surface free energy γS is greater than 0 and no greater than 1 mN/m, and γSh (hydrogen bonding force component of surface free energy) forming the surface free energy γS is greater than 0 and no greater than 6 mN/m.

This application claims priority to prior Japanese Patent ApplicationNo. 2004-30295, the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

This invention relates to a magnetic disk for use in a magneticrecording device (HDD, Hard Disk Drive) or the like and a manufacturingmethod thereof. More specifically, this invention relates to a magneticdisk that can prevent a fly stiction trouble, a corrosion failure, andso on to suppress a malfunction and is thus excellent in safety evenwhen a magnetic head performs a flying operation at a flying height of,for example, 10 nm or less, and a manufacturing method of the magneticdisk.

An LUL (Load Unload) system capable of higher recording capacity hasstarted to be employed in HDDs (Hard Disk Drives) in recent years. Inthe LUL system, upon halting, a magnetic recording head is retreated toa slope bed called a ramp located outside a magnetic disk while, uponstarting, the magnetic recording head is, after the magnetic disk startsrotation, slided from the ramp over the surface of the magnetic disk,and then recording/reproduction is carried out. Therefore, the magneticrecording head does not contact and slide on the magnetic disk.

In this LUL system, it is not necessary to provide a contact/slideregion (CSS zone) for the magnetic recording head on the surface of themagnetic disk, which is required in the CSS system conventionally used.Therefore, it is possible to ensure a wider area of arecording/reproduction region as compared with the CSS system and thusincrease the recording capacity of the magnetic recording medium.Further, in the LUL system, since the magnetic disk and the magneticrecording head do not contact each other, it is not required to providetexture for preventing contact adsorption, which is required in the CSSsystem, so that the surface of the magnetic disk can be made evensmoother. Therefore, the recording density of the magnetic disk can beincreased by reducing the flying height of the magnetic recording headas compared with the CSS system.

As such a magnetic disk, there is known a magnetic recording medium asdisclosed in, for example, Japanese Unexamined Patent ApplicationPublication (JP-A) No. 2003-248917.

On the other hand, in a magnetic recording device (HDD), volatileorganic gases such as sulfur-based organic compound, chlorine-basedorganic compound, dioctyl phthalate, acrylic acid, and siloxane, acidgas, and so on are emitted at a certain ratio from various organicmaterials such as adhesives and plastic materials that are used in thedevice. Therefore, the organic gas, the acid gas, or the like tends tobe adsorbed to a magnetic recording medium in an environment of, forexample, high temperature and high humidity. Further, interaction occursbetween the adsorbed gas and a lubricant of a lubrication layer so thatthe lubrication layer is liable to change in quality.

Such problems are becoming remarkable particularly following a reductionin flying height of a magnetic head. When the magnetic head flies overthe surface of a magnetic disk at a low flying height (e.g. a smallflying height of 10 nm or less), the magnetic head gathers up theorganic compound etc. and the lubricant adsorbed on the surface of themagnetic disk, which tend to be transferred and deposited on the surfaceof the magnetic head. Particularly, in the case of a magnetic headhaving an NPAB slider (negative pressure slider), a strong negativepressure occurs at a lower surface (surface on the side of the magneticdisk) of the magnetic head to gather up, like a vacuum cleaner, theorganic compound etc. and the lubricant adsorbed on the surface of themagnetic disk so that they tend to be transferred and deposited on thesurface of the magnetic head.

If this transfer state exceeds a certain level, a trouble called a flystiction phenomenon and a corrosion failure occur. The fly stiction is atrouble where the flying posture and height of the magnetic recordinghead go out of order during its flying operation and irregular changesin reproduction output occur frequently. According to circumstances, themagnetic recording head is brought into contact with the magneticrecording medium to crash during the flying operation, therebydestroying the magnetic disk. This fly stiction often occurs without apremonitory sign and is one of troubles that are difficult to control.

In the conventional CSS system, the CSS operation at the time ofstarting and stopping serves to perform cleaning of the lubricant andthe organic compound etc. transferred to the magnetic recording headand, therefore, those troubles do not raise a problem.

On the other hand, in the LUL system, since there is no sliding movementbetween the magnetic recording medium and the magnetic recording head,there is no function of cleaning the lubricant and the organic compoundetc. transferred to the head. Therefore, particularly in the LUL system,the lubricant and the organic compound etc. tend to be transferred anddeposited on the magnetic recording head so that the fly stiction isliable to occur and a head element portion is liable to be corroded.Further, if this deposition advances, it often drops on the surface ofthe medium as deposits to damage a protection film to thereby enablerecording and reproduction. Since the flying height of the magneticrecording head has still been reduced (10 nm or less) following theshift to the LUL system from the CSS system, the transfer and depositiononto the head has been further facilitated.

In addition, recently, magnetic disk devices such as HDDs (hard diskdrives) have been often used in environments of low atmosphericpressures such as in an airplane. Following it, there has been arising aproblem about flying stability of heads. Specifically, the flying heightof the magnetic head further decreases from 10 nm due to a change inatmospheric pressure and, further, there occurs variation in flyingheight due to processing accuracy of air bearing sliders of the magneticheads. As a result, a problem of the fly stiction has occurredfrequently.

Further, recent magnetic disk devices such as HDDs (Hard Disk Drives)have been miniaturized and incorporated into digital cameras and musicreproducing players. Under these circumstances, external pressures suchas any environments (in airplane as described before, mountaintop, hightemperature, low temperature, high humidity, low humidity) and any usingmanners (desktop, portable) affect flying properties (stable flyingperformance) of heads in magnetic disk devices and outgases from membersin the magnetic disk devices.

As a cause for occurrence of the fly stiction, there can be consideredthe influence of roughness of the surface of the magnetic disk,interaction (meniscus force) between the lubrication layer and the head,or contamination due to outgas from the magnetic disk device.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a magnetic diskthat can prevent a fly stiction trouble, a corrosion failure, and so onto suppress a malfunction and is thus excellent in safety even when amagnetic head performs a flying operation at a flying height of, forexample, 10 nm or less and, in particular, the magnetic disk that issuitable for an HDD of the LUL system.

It is another object of this invention to provide a method ofefficiently manufacturing the magnetic disk.

In order to achieve the foregoing objects, the present inventor hasstudied, with respect to the fly stiction phenomenon liable to occurparticularly in the LUL system, about causal relation to a lubricant,thickness of the lubricant, adhesion of the lubricant, a protectionlayer, shape of a magnetic recording head, flying height thereof, and soon. As a result, the present inventor has found that the fly stictionphenomenon has a close relationship with surface energy of the surfaceof a magnetic disk.

That is, the present inventor has found that it is effective to the flystiction to reduce the surface energy of the magnetic disk surface tothereby minimize organic matter adhering to the outermost surface of themagnetic disk.

Specifically, the present inventor has found that, in order to preventthe fly stiction trouble and the corrosion failure and achieve excellentLUL durability, it is important to inactivate the outermost surface ofthe magnetic disk to a predetermined degree. The present inventor hasunderstood that, in this event, it is not sufficient to merely specifydesired values of surface free energy and critical surface tension ofthe magnetic disk surface, but it is necessary to simultaneously specifyrespective components forming them.

Substances adherable or adsorbable to the magnetic disk surface andlubricants are diversified. Therefore, although merely referring to thedegree of inactivation or the desired value of the surface free energy,the desired value differs per substance or compound. However, it ispractically impossible to assume, in advance, all individual substancesor compounds that are adherable or adsorbable to the magnetic disksurface and specify the degrees of inactivation with respect to allthose substances.

The present inventor has found that if it is possible to decomposesurface free energy into respective components paying attention to itsproperties and set desired values of the respective components toprescribed values that are preferable to solve the problems for thisinvention, the problems can be solved without the necessity forspecifying the inactivity degrees about individual substances one by onein advance.

Further, the present inventor has found that it is effective to treatthe magnetic disk surface by the use of a composition containinghydrofluoroether, as means for specifying surface free energy andcritical surface tension, specified by this invention, to desiredvalues. Specifically, a treatment is carried out so that the compositioncontaining hydrofluoroether is brought into contact with the surface ofa magnetic disk that has finally been formed with a lubrication layer.

In the process of forming a lubrication layer in a magnetic disk,preparation is made of a solution in which a perfluoropolyether-basedlubricant is dispersed and dissolved in a fluorine-based solvent such asHFC (hydrofluorocarbon) or PFC (perfluorocarbon). A magnetic diskfinally formed with a protection layer is dipped into this solution toperform application by a dipping method or the like, thereby forming thelubrication layer.

A perfluoropolyether-based lubricant for use in a magnetic disk has amain chain portion containing fluorine and polar groups as functionalgroups at ends of this main chain. As the polar groups, use is made of,for example, hydroxyl groups, carboxyl groups, or other polar groups.Since the main chain portion of the lubricant has a flexible structure,the lubricant exhibits suitable lubrication performance, while the polargroups at the end portions exhibit an action of adhering to a protectionlayer by intermolecular force or the like. Therefore, this lubricant isfixed on the magnetic disk as a lubrication layer (film). Normally, ithas two to four polar groups as functional end groups.

The present inventor has paid attention to the presence of these polargroups. That is, if the polar groups as end groups of the lubricantforming the lubrication layer are completely oriented toward the side ofthe protection layer, the surface of the lubrication layer is coveredwith main chains (fluorine etc.) of perfluoropolyether. As a result, itis expected that the magnetic disk surface exhibits very low surfacefree energy and very low critical surface tension. However, actually,because of orientation of a substance forming the protection layer andorientation of the lubricant, a cubic structure of the lubricant itself,and so on, the polar groups being the end groups are not completelyoriented toward the side of the protection layer. Therefore, it isconsidered that a certain amount of polar groups are exposed on themagnetic disk surface after the formation of the lubrication layer.

Because of the exposure of the polar groups on the lubrication layersurface, i.e. the magnetic disk surface, surface free energy andcritical surface tension of the magnetic disk surface become large. Thisis considered to prevent desired inactivation. Therefore, the presentinventor has paid attention to an idea that if a treatment is performedto inactivate the polar groups, being the functional groups of thelubricant, exposed on the surface, desired inactivity degrees may beobtained.

It is considered that this mechanism is derived from a structure of HFE(hydrofluoroether). That is, HFE has a structural formula ofC_(n)F_(2n+1)—O—R (R═C_(n)H_(2n+1)) and has an ether bond betweenC_(n)F₂₊₁ group and R (R═C_(n)H_(2n+1)) group. When the treatment isperformed to contact hydrofluoroether with the surface of the magneticdisk finally formed with the lubrication layer, those ether bond groupsare bonded, by intermolecular force or hydrogen bonding force, to thefunctional end groups (polar groups) of the perfluoropolyether lubricantthat are not oriented toward the protection layer surface. As a result,the outermost surface of the magnetic disk is considered to besubstantially C_(n)F_(2n+1) groups and R (R═C_(n)H_(2n+1)) groups.Accordingly, it is considered that the exposed polar groups can beinactivated.

This invention has been completed on the basis of the foregoingknowledge.

Specifically, this invention has the following structures.

(Structure 1)

A magnetic disk having a magnetic layer, a protection layer, and alubrication layer formed on a substrate in this order, wherein a surfacefree energy γS of a surface of the magnetic disk derived by an extendedFowkes equation is greater than 0 and no greater than 24 mN/m, γSd(dispersion force component of surface free energy) forming the surfacefree energy γS is greater than 0 and no greater than 17 mN/m, γSp(dipole component of surface free energy) forming the surface freeenergy γS is greater than 0 and no greater than 1 mN/m, and γSh(hydrogen bonding force component of surface free energy) forming thesurface free energy γS is greater than 0 and no greater than 6 mN/m(hereinafter referred to as a magnetic disk I).

(Structure 2)

A magnetic disk having a magnetic layer, a protection layer, and alubrication layer formed on a substrate in this order, wherein a surfacefree energy γS of a surface of the magnetic disk derived by a Van-Ossequation is greater than 0 and no greater than 22 mN/m, γSLW forming thesurface free energy γS is greater than 0 and no greater than 17 mN/m,γS⁻ forming the surface free energy γS is greater than 0 and no greaterthan 6 mN/m, and γS⁺ forming the surface free energy γS is greater than0 and no greater than 1 mN/m (hereinafter referred to as a magnetic diskII).

(Structure 3)

A magnetic disk having a magnetic layer, a protection layer, and alubrication layer formed on a substrate in this order wherein, acritical surface tension γc of a surface of the magnetic disk derived bya Zisman equation is greater than 0 and no greater than 17 mN/m(hereinafter referred to as a magnetic disk III).

(Structure 4)

A magnetic disk of structure 1 or 2, wherein a critical surface tensionγc of the surface of the magnetic disk derived by a Zisman equation isgreater than 0 and no greater than 17 mN/m.

(Structure 5)

A manufacturing method of a magnetic disk having a magnetic layer, aprotection layer, and a lubrication layer formed on a substrate in thisorder, comprising treating a surface of the magnetic disk by the use ofa composition containing hydrofluoroether after formation of thelubrication layer.

(Structure 6)

A manufacturing method of a magnetic disk of structure 5, wherein, afterforming the lubrication layer on a surface of the protection layer, thedisk is heated in a clean room before and/or after the treatment by theuse of the composition containing hydrofluoroether.

(Structure 7)

A manufacturing method of a magnetic disk of structure 5 or 6, whereinthe hydrofluoroether has a molecular weight of 150 to 400.

(Structure 8)

A manufacturing method of a magnetic disk of structure 5 or 6, whereinthe lubrication layer is formed by a film of a perfluoropolyethercompound having polar groups at ends.

(Structure 9)

A manufacturing method of a magnetic disk of structure 5 or 6, whereinthe protection layer is an amorphous carbon protection layer formed by aplasma CVD method.

According to this invention, even when a magnetic head performs a flyingoperation at a flying height of, for example, 10 nm or less, a flystiction trouble, a corrosion failure, and so on can be prevented sothat a malfunction is suppressed. As a result, it is possible to providea magnetic disk excellent in safety, particularly the magnetic disksuitable for an HDD of the LUL system, and a method of efficientlymanufacturing such a magnetic disk.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE is a sectional view exemplarily showing one example of alayer structure of a magnetic disk of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A magnetic disk of this invention is a magnetic disk having a magneticlayer, a protection layer, and a lubrication layer that are formed inthis order.

The protection layer is not particularly limited but is preferably acarbon-based protection layer. As a structure, it is preferably aprotection layer made of amorphous carbon. Specifically, it can be anamorphous diamond-like carbon protection layer. By the use of such acarbon-based protection layer, suitable LUL durability can be obtained.

In this invention, the thickness of the carbon-based protection layer ispreferably 1 to 5 nm. When less than 1 nm, there is a problem inabrasion resistance. Although it is not necessary to particularlyprovide an upper limit to the thickness of the carbon-based protectionlayer, it is preferably set to 5 nm or less on a practical basis so asnot to impede the improvement in magnetic spacing.

Further, in the case of the carbon-based protection layer, it is, as acomposition, preferably a hydrogenated carbon protection layercontaining hydrogen. By the use of hydrogenated carbon, the protectionperformance becomes high with a fine structure and therefore it issuitable as a magnetic disk for the LUL system. In this case, it ispreferable that the content of hydrogen be set to 3 atm % or more andless than 20 atm % relative to the total of the carbon-based protectionlayer when measuring it by HFS (Hydrogen Forward Scattering). When thecontent of hydrogen relative to the carbon-based protection layer isless than 3 atm %, the fineness or hardness is often lowered andtherefore the magnetic layer cannot be suitably protected from animpulsive force upon the start of LUL. On the other hand, when thecontent of hydrogen is 20 atm % or more, polymeric carbon componentsincrease. As a result, adhesion performance of the protection layer withrespect to the magnetic layer is reduced and therefore the protectionlayer is often stripped off upon the start of LUL, which is thus notpreferable.

It is further preferable to use a carbon nitride protection layer or ahydrogenated carbon nitride protection layer which is in the form of acarbon-based protection layer containing nitrogen. This is because, bythe inclusion of nitrogen, it is possible to highly promote orientationof polar end groups of the lubricant toward the side of the protectionlayer. Therefore, when combined with this invention, it is possible toobtain a particularly preferable effect. The content of nitrogenrelative to carbon can be 4 to 12 atm % when measured by XPS (X-rayPhotoelectron Spectroscopy).

In this invention, the carbon-based protection layer is preferably aprotection layer formed by CVD (Chemical Vapor Deposition).Particularly, it is preferable that the carbon-based protection layer beformed by plasma CVD (P-CVD) that uses plasma to excite atoms. This isbecause the carbon-based protection layer formed by P-CVD is high infineness and hardness and thus excellent in LUL durability. When formingthe carbon-based protection layer by P-CVD, it is preferable thatdiamond-like carbon is formed using a hydrocarbon gas as a reactive gas.

As the reactive gas, it is preferable to use low-grade hydrocarbon.Particularly, use is preferably made of any of low-grade saturatedhydrocarbon, low-grade unsaturated hydrocarbon, and low-grade ringhydrocarbon. As the low-grade saturated hydrocarbon, use can be made ofmethane, ethane, propane, butane, octane, or the like. As the low-gradeunsaturated hydrocarbon, use can be made of ethylene, propylene,butylene, acetylene, or the like. As the low-grade ring hydrocarbon, usecan be made of benzene, toluene, xylene, styrene, naphthalene,cγclohexane, or the like. Note that low-grade referred to hereinrepresents hydrocarbon where the number of carbons per molecule is 1 to10. The reason why use of the low-grade hydrocarbon is preferable isthat as the number of carbons increases, it becomes difficult tovaporize it into gas and supply it to a film-forming apparatus and, inaddition, it becomes difficult to decompose it during plasma discharge.Further, when the number of carbons increases, high-molecularhydrocarbon components tend to be largely contained in components of theformed protection layer to thereby reduce the fineness and hardness ofthe protection layer, which is thus not preferable. In view of this, itis preferable to use the low-grade hydrocarbon as hydrocarbon. It isparticularly preferable to use acetylene because a fine andhigh-hardness carbon-based protection layer can be formed.

In this invention, the lubrication layer is preferably a layer in theform of a film of a perfluoropolyether lubricant having hydroxyl groupsas end groups. Particularly, it is preferably a perfluoropolyethercompound refined in a predetermined manner by the use of a supercriticalfluid extraction method. The main chain of the perfluoropolyethercompound has a straight chain structure to thereby exhibit properlubrication performance for the magnetic disk and, because of includinghydroxyl groups (OH), as polar groups, being end groups, it candemonstrate high adhesion performance with respect to the carbon-basedprotection layer. Particularly, in the case of containing nitrogen onthe surface of the carbon-based protection layer, since (N⁺) and (OH⁻)exhibit high affinity, high adhesion rate of the lubrication layer canbe achieved, which is thus preferable.

As the perfluoropolyether compound having hydroxyl groups as end groups,it is preferable that the number of hyroxyl groups per molecule be 2 to4. When less than two, there is a case where the adhesion rate of thelubrication layer is lowered, which is thus not preferable. Whenexceeding four, there is a case where the lubrication performance isreduced due to excessive improvement in adhesion rate. The thickness ofthe lubrication layer may be properly adjusted within the range of 0.5to 1.5 nm. When less than 0.5 nm, there is a case where the lubricationperformance is reduced, while, when exceeding 1.5 nm, there is a casewhere the adhesion rate of the lubrication layer is lowered.

In this invention, it is preferable that, after forming the lubricationlayer, the surface of the lubrication layer, i.e. the surface of themagnetic disk, be treated with HFE (hydrofluoroether). Specifically, itis preferable to perform a treatment where HFE (hydrofluoroether) isbrought into contact with the magnetic disk having been formed with thelubrication layer. For example, the treatment can be carried out by avapor deposition method, an immersion method, or the like. Through thetreatment in this manner, HFE (hydrofluoroether) is formed as a film onthe surface of the lubrication layer of the magnetic disk.

The hydrofluoroether compound used in the HFE (hydrofluoroether)treatment is preferably a compound having a molecular weight of about150 to 400. Particularly, it is preferable to select a compound having amolecular weight of 350 or less. Specifically, use can be preferablymade of C₄F₉—O—CH₃ and/or C₄F₉—O—C₂H₅. Further, the surface tension ofhydrofluoroether is preferably set greater than 0 and no greater than 14mN/m.

According to this invention, it is preferable to apply a heat treatmentto the magnetic disk in a clean room after the formation of thelubrication layer. In this event, the clean degree is set to a cleanatmosphere equal to or higher than class 6 defined in JapaneseIndustrial Standard (JIS) B9920. By performing the heat treatment in aclean atmosphere of class 1 to 6, it is possible to promote orientationof the end polar groups of perfluoropolyether main chains toward theprotection layer. The heat treatment temperature can be set in the rangeof about 80° C. to 150° C. The heat treatment can be carried out beforeand/or after the HFE (hydrofluoroether) treatment, after the formationof the lubrication layer. Preferably, it is carried out after the HFE(hydrofluoroether) treatment.

In this invention, a glass substrate is preferably used as a substrate.The glass substrate can achieve smoothness and high rigidity. Therefore,the magnetic spacing, particularly the flying height of the magnetichead, can be further reduced stably, which is thus particularlypreferable in this invention. As a material of the glass substrate,aluminosilicate glass is particularly preferable. Aluminosilicate glasscan achieve high rigidity and strength by chemical strengthening.

In this invention, the surface roughness of the magnetic disk surface ispreferably 4 nm or less at Rmax and 0.4 nm or less at Ra. When Rmaxexceeds 4 nm, the reduction in magnetic spacing is often avoided, whichis thus not preferable. It is noted here that the surface roughnessreferred to herein is defined in Japanese Industrial Standard (JIS)B0601.

Further, in this invention, a CoPt-based ferromagnetic layer can be usedas the magnetic layer being a magnetic recording layer. The magneticlayer can be formed on the substrate by a film forming method such as asputtering method.

Next, in the magnetic disk I of this invention, in order to achieve theobjects of this invention, it is necessary to satisfy the condition thata surface free energy γS of the magnetic disk surface derived by anextended Fowkes equation, which will be explained below, is greater than0 and no greater than 24 mN/m, γSd (dispersion force component ofsurface free energy) forming the surface free energy γS is greater than0 and no greater than 17 mN/m, γSp (dipole component of surface freeenergy) forming the surface free energy γS is greater than 0 and nogreater than 1 mN/m, and γSh (hydrogen bonding force component ofsurface free energy) forming the surface free energy γS is greater than0 and no greater than 6 mN/m.

<Extended Fowkes Equation>

This is a theory proposed by Fowkes in 1964. When the surface tension islargely divided into a dispersion component (only London force) and apolar component (including Debye force and hydrogen bonding force),γ=γd+γp   (1)

γd dispersion component

γp: polar component

Here, given that WSL: the work of adhesion between solid and liquid,WSL=WSLd+WSLp   (2)

WSLd : dispersion component of adhesion work

WSL: polar component of adhesion work

Here, when a geometric average is calculated only with respect to thedispersion component of liquid and solid from (2),WSLd=(γSd·γLd)0.5   (3)

From this equation and the Dupre-Young equation,γL(1+cos θ)=2(γSd·γLd)0.5   (4)

From equation (4), γSd can be calculated.

Note that θ is a contact angle between a liquid and a solid body surface(the same shall apply hereinafter).

Fowkes gives consideration only to the dispersion force as aninteraction. On the other hand, in this invention, use is made of anextended Fowkes equation that takes into account a surface interactionforce γSp caused by an intermolecular force based on polarity such as apermanent polar effect and an induced polar effect, and a hydrogenbonding interaction force γSh. $\begin{matrix}{{WAB} = {\gamma\quad{L\left( {1 + {\cos\quad\theta}} \right)}}} \\{= {{2\left( {\gamma\quad{{Sd} \cdot \gamma}\quad{Ld}} \right)0.5} + {2\left( {\gamma\quad{{Sp} \cdot \gamma}\quad{Lp}} \right)0.5} +}} \\{2\left( {\gamma\quad{{Sh} \cdot \gamma}\quad{Lh}} \right)0.5}\end{matrix}$ $\begin{matrix}{{\cos\quad\theta} = \left\{ {{2\left( {\gamma\quad{{Sd} \cdot \gamma}\quad{Ld}} \right)0.5} + {2\left( {{\gamma\quad{Sp}}{{\cdot \gamma}\quad{Lp}}} \right)0.5} +} \right.} \\{{{\left. {2\left( {\gamma\quad{{Sh} \cdot \gamma}\quad{Lh}} \right)0.5} \right\}/\gamma}\quad L} - 1}\end{matrix}$

By measuring by the use of liquids of which γLd, γLp, and γLh are known,the foregoing equations become simultaneous equations so that γSd, γSp,and γSh can be determined.

On the other hand, in the magnetic disk II of this invention, in orderto achieve the objects of this invention, it is necessary to satisfy thecondition that a surface free energy γS of the magnetic disk surfacederived by a Van-Oss equation, which will be explained below, is greaterthan 0 and no greater than 22 mN/m, γSLW forming the surface free energyγS is greater than 0 and no greater than 17 mN/m, γS⁻ forming thesurface free energy γS is greater than 0 and no greater than 6 mN/m, andγS⁺ forming the surface free energy γS is greater than 0 and no greaterthan 1 mN/m.

<Van-Oss Equation>

This is a theory proposed by Van Oss in 1987 giving consideration to anacid-base interaction.γ=γLW+γab   (7)

γLW: Lifshitz-Van der Waals force (including London force and Debye andKeesom force)

γab: acid-base interaction

Van Oss takes γab as a geometric average of a giving component γ⁻ and areceiving component γ⁺ and derives equation (8).γ=γLW+2(γ⁻·γ⁺)0.5   (8)

From equations (3) and (4) of adhesion work,WSLd=(γSd·γLd)0.5   (3)

from this equation and the Dupre-Young equation, $\begin{matrix}{{\gamma\quad{L\left( {1 + {\cos\quad\theta}} \right)}} = {2\left( {\gamma\quad{{Sd} \cdot \gamma}\quad{Ld}} \right)0.5}} & (4) \\\begin{matrix}{{WAB} = {\gamma\quad{L\left( {1 + {\cos\quad\theta}} \right)}}} \\{= {{2\left( {\gamma\quad{{SLW} \cdot \gamma}\quad{LLW}} \right)0.5} + {2\left( {\gamma\quad{S^{-} \cdot \gamma}\quad L^{+}} \right)0.5} +}} \\{2\left( {\gamma\quad{S^{+} \cdot \gamma}\quad L^{-}} \right)0.5}\end{matrix} & (9)\end{matrix}$

By rearranging equation (9),cos θ={2(γSLW·γLLW)0.5+2(γS ⁻ γL ⁺)0.5+2(γS⁺ γL ⁻)0.5}/γL−1   (10)

Further, γS isγS=γSLW+γSA·B=γSLW+2√(γS ⁺ ·γS ⁻)[γSA·B is a surface energy of acid-base interaction andγSA·B=2√(γS⁺·γS⁻)]

Therefore, by measuring by the use of liquids of which γL, γLLW, γL⁺,and γL⁻ are known, γSLW, γS⁻, γS⁺, and γS can be determined.

Further, in the magnetic disk III of this invention, in order to achievethe objects of this invention, it is necessary to satisfy the conditionthat a critical surface tension γc of the magnetic disk surface derivedby a Zisman equation, which will be explained below, is greater than 0and no greater than 17 mN/m.

<Zisman Equation>

This is widely used as a technique for calculating a critical surfacetension (γc). By the use of a plurality of liquids each having only aVan der Waals force on a solid body surface and having a known surfacetension, angles (contact angles θ) formed between the liquids and thesolid body surface immediately after dropping of the liquids aremeasured, respectively. When plotting the surface tensions of theliquids on x-axis and cos θ on y-axis, a straight line descendingrightward is obtained (Zisman Plot). The surface tension when Y=1 (θ=0)on this straight line is calculated as a critical surface tension γc.

It is preferable that the magnetic disks I and II respectively satisfythe condition of the magnetic disk III.

These magnetic disks of this invention each can be suitably used as amagnetic disk for use in an HDD of the LUL system.

FIG. 1 is a sectional view exemplarily showing one example of a layerstructure of a magnetic disk of this invention. This magnetic disk 10comprises at least a substrate 1, a magnetic layer 3 formed on thesubstrate, a protection layer 4 formed on the magnetic layer 3, and alubrication layer 5 formed on the protection layer 4. In this example,the magnetic layer 3 and the protection layer 4 are formed so as tocontact each other and the protection layer 4 and the lubrication layer5 are formed so as to contact each other.

Between the substrate 1 and the magnetic layer 3, a nonmagnetic metallayer 2 comprising a seed layer 2 a and an underlayer 2 b is formed. Inthe magnetic disk 10, all except the magnetic layer 3 are nonmagneticsubstances.

Further, this invention also provides a manufacturing method of themagnetic disk 10 having the magnetic layer 3, the protection layer 4,and the lubrication layer 5 formed on the substrate 1 in this order. Thesurface of the magnetic disk is treated with a composition containinghydrofluoroether after the formation of the lubrication layer 5.

In the manufacturing method of the magnetic disk of this invention, asdescribed before, it is preferable that, after forming the lubricationlayer 5 on the magnetic disk surface, the magnetic disk 10 be heated ina clean room before and/or after the treatment by the use of thecomposition containing hydrofluoroether. Hydrofluoroether preferably hasa molecular weight of 150 to 400. Further, it is preferable that thelubrication layer 5 be formed by a film of a perfluoropolyether compoundhaving polar groups at ends. The protection layer 4 is preferably anamorphous carbon protection layer formed by plasma CVD.

EXAMPLES

Now, this invention will be described in further detail by the use ofexamples, but this invention is not limited at all by those examples.

Example 1

A magnetic disk 10 for the LUL system having a structure shown in FIG. 1was manufactured.

First, aluminosilicate glass was formed into a disk shape to obtain aglass disk. By applying grinding, precision polishing, end-facepolishing, precision cleaning, and chemical strengthening to theobtained glass disk, a flat, smooth, and high-rigidity glass substrate 1for a magnetic disk was obtained. This glass substrate 1 was a 2.5-inchmagnetic disk substrate having a diameter of 65 mm, an inner diameter of20 mm, and a disk thickness of 0.635 mm.

Here, observing the surface roughness of the obtained glass substrate 1by the use of an AFM (Atomic Force Microscope), it was confirmed to be asmooth surface having Rmax of 3.96 nm and Ra of 0.36 nm.

Then, by the use of a static opposed type film-forming apparatus, a seedlayer 2 a, an underlayer 2 b, and a magnetic layer 3 were formed on theglass substrate 1 in this order by DC magnetron sputtering.Specifically, first, using an AlRu (Al: 50 atm %, Ru: 50 atm %) alloy asa sputtering target, the seed layer 2 a made of the AlRu alloy andhaving a thickness of 30 nm was formed on the glass substrate 1 bysputtering. Then, using a CrMo (Cr: 80 atm %, Mo: 20 atm %) alloy as asputtering target, the underlayer 2 b made of the CrMo alloy and havinga thickness of 20 nm was formed on the seed layer 2 a by sputtering.Then, using a CoCrPtB (Cr: 20 atm %, Pt: 12 atm %, B: 5 atm %, theremainder: Co) alloy as a sputtering target, the magnetic layer 3 havinga thickness of 6 nm was formed on the underlayer 2 b. This magneticlayer 3 serves for magnetic recording.

Then, on the disk having been formed with the magnetic layer 3, acarbon-based protection layer 4 made of carbon, hydrogen, and nitrogenwas formed by the use of plasma CVD (P-CVD). Specifically, using a mixedgas in the form of a mixture of acetylene and nitrogen in the ratio of97%:3% as a reactive gas, deposition was carried out so that thecarbon-based protection layer 4 by plasma CVD having a thickness of 4.5nm was formed on the magnetic layer 3. The deposition rate at the timeof forming the carbon-based protection layer was 1 nm/s. In theformation of the protection layer, high frequency power (frequency 27MHz) was applied to electrodes to produce plasma. Further, a bias of−300 W was applied. As the thickness of the protection layer 4, theactual thickness was measured through cross-section observation by atransmission electron microscope (TEM). Incidentally, in this event,P-CVD film formation may be implemented as IBD (ion Beam Deposition) byapplying a voltage to plasma, or the like.

The formed carbon-based protection layer 4 was examined and confirmed tobe an amorphous diamond-like carbon protection layer. The compositionwas examined and it was hydrogenated carbon nitride. The content ofhydrogen was examined by HFS (Hydrogen Forward Scattering) and hydrogenwas contained at about 15 atm % relative to hydrogenated carbon nitride.The content of nitrogen was examined by XPS (X-ray PhotoelectronSpectroscopy) and nitrogen was contained at 8 atm % relative to carbon.

Then, after the formation of the carbon-based protection layer 4, themagnetic disk surface was cleaned by the use of heated ultrapure waterand then further cleaned by the use of isopropyl alcohol, and thensubjected to finish drying. Then, by the use of a dipping method, alubrication layer 5 made of a PFPE (perfluoropolyether) compound wasformed on the carbon-based protection layer 4. Specifically, use wasmade of an alcohol denatured perfluoropolyether lubricant havinghydroxyl groups as polar groups at both ends of the main chain ofperfluoropolyether. For the purpose of removing impurities and so on,use was made of the lubricant refined by the use of a supercriticalfluid extraction method. After the formation of the lubrication layer 5by the use of the dipping method, drying was carried out.

Then, the surface treatment was performed by the use of HFE(hydrofluoroether). Specifically, use was made of a liquid compositioncomposed of a hydrofluoroether compound having a C₄F₉—O—CH₃ structure.The molecular weight of this hydrofluoroether is 250. Further, thesurface tension is 13.6 mN/m. By depositing this hydrofluoroethercomposition on the surface of the lubrication layer 5 by the use of avapor deposition method (treatment time 60 seconds), hydrofluoroetherwas brought into contact with the magnetic disk surface.

Then, the magnetic disk 10 was subjected to a heat treatment at 110° C.for 60 seconds. In this event, the heat treatment was performed in anatmosphere of clean degree class 5 of a clean environment defined inJapanese Industrial Standard (JIS) B9920.

In this manner, the magnetic disk 10 was manufactured. The thickness ofthe lubrication layer 5 after burning was 1.2 nm. Observing the surfaceroughness of the obtained magnetic disk 10 by the use of the AFM, it wasconfirmed to be a smooth surface having Rmax of 4 nm and Ra of 0.4 nm.

Further, the glide height was measured to be 4.5 nm. In order to stablyachieve a flying height of the magnetic head being 10 nm or less, theglide height of the magnetic disk is preferably set to 5 nm or less.

With respect to the obtained magnetic disk 10, various performances wereevaluated and analyzed in the following manner.

(1) Surface Tension Measurement

(a) Extended Fowkes Equation

Tetradecane (surface energy; γLd 26.7 mN/m, γLp 0 mN/m, γLh 0 mN/m, γL26.7 mN/m), methylene iodide (surface energy; γLd 46.8 mN/m, γLp 4 mN/m,γLh 0 mN/m, γL 50.8 mN/m), and water (surface energy; γLd 29.1 mN/m, γLp1.3 mN/m, γLh 42.4 mN/m, γL 72.8 mN/m) were used as liquids and angles(contact angles) between the liquids and the solid body surface in themagnetic disk obtained as described above were measured. The resultswere tetradecane 60.0°, methylene iodide 86.8°, and water 93.5°.

First, in the case of having used tetradecane, γSd was derived.$\begin{matrix}{{\cos\quad\left( {60{^\circ}} \right)} = \left\{ {{2\left( {\gamma\quad{{Sd} \cdot 26.7}} \right)0.5} + {2\left( {{\gamma\quad{Sp}}{\cdot 0}} \right)0.5} +} \right.} \\{{\left. {2\left( {\gamma\quad{{Sh} \cdot 0}} \right)0.5} \right\}/26.7} - 1}\end{matrix}$

Since the second and third terms are 0, γSd=15.02 mN/m.

Then, in the case of having used methylene iodide, γSp was derived.$\begin{matrix}{{\cos\quad\left( {86.8{^\circ}} \right)} = \left\{ {{2\left( {15.02 \cdot 46.8} \right)0.5} + {2\left( {{\gamma\quad{Sp}}{\cdot 4}} \right)0.5} +} \right.} \\{{\left. {2\left( {\gamma\quad{{Sh} \cdot 0}} \right)0.5} \right\}/50.8} - 1}\end{matrix}$

Since the third term is 0, γSp=0.023 mN/m.

Then, in the case of having used water, γSh was derived. $\begin{matrix}{{\cos\quad\left( {93.5{^\circ}} \right)} = \left\{ {{2\left( {15.02 \cdot 29.1} \right)0.5} + {2\left( {0.023{\cdot 1.3}} \right)0.5} +} \right.} \\{{\left. {2\left( {\gamma\quad{{Sh} \cdot 42.4}} \right)0.5} \right\}/72.8} - 1}\end{matrix}$

Thus, γSh=4.07 mN/m.

Therefore, from γS=γSd+γSp+γSh,

γS=19.11 mN/m.

(b) Van Oss Equation

Tetradecane (surface energy; γL 26.7 mN/m, γLLW 26.7 mN/m, γL⁺ 0.0 mN/m,γL⁻ 0.0 mN/m), water (surface energy; γL 72.8 mN/m, γLLW 21.8 mN/m, γL⁺25.5 mN/m, γL⁻ 25.5 mN/m), and ethylene glycol (surface energy; γL 48.0mN/m, γLLW 29.0 mN/m, γL⁺ 1.92 mN/m, γL⁻ 47.0 mN/m) were used as liquidsand contact angles between the liquids and the solid body surface in themagnetic disk obtained as described above were measured. The resultswere tetradecane 60.0°, water 93.5°, and ethylene glcol 74.5°.

First, in the case of having used tetradecane, γSLW was derived.$\begin{matrix}{{\cos\quad\left( {60{^\circ}} \right)} = \left\{ {{2\left( {\gamma\quad{{SLW} \cdot 26.7}} \right)0.5} + {2\left( {0 \cdot 0} \right)0.5} +} \right.} \\{{\left. {2\left( {0 \cdot 0} \right)0.5} \right\}/26.7} - 1}\end{matrix}$

Therefore, γSLW=15.02 mN/m.

Similarly, in the case of having used water, $\begin{matrix}{{\cos\quad\left( {93.5{^\circ}} \right)} = \left\{ {{2\left( {15.02 \cdot 21.8} \right)0.5} + {2\left( {{\gamma S}^{-}{\cdot 25.5}} \right)0.5} +} \right.} \\{{\left. {2\left( {\gamma\quad{S^{+} \cdot 25.5}} \right)0.5} \right\}/72.8} - 1}\end{matrix}$

Similarly, in the case of having used ethylene glycol, $\begin{matrix}{{\cos\quad\left( {74.5{^\circ}} \right)} = \left\{ {{2\left( {15.02 \cdot 29.0} \right)0.5} + {2\left( {{\gamma S}^{-}{\cdot 1.92}} \right)0.5} +} \right.} \\{{\left. {2\left( {\gamma\quad{S^{+} \cdot 47.0}} \right)0.5} \right\}/48} - 1}\end{matrix}$

From the above two simultaneous equations,γS⁻=5.07 mN/m and γS⁺=0.88 mN/m.

Further, γS isγS=γSLW+γSA·B=γSLW+2√(γS ⁺ ·γS ⁻)

Therefore,γS=15.02+2(5.07×0.88)0.5=19.25 mN/m

(c) Zisman Equation

Nonpolar (Van der Waals) alkanes having the following surface tensionswere used as liquids and contact angles between the liquids and thesolid body surface in the foregoing magnetic disk were derived. surfacetension contact angle [θ] (mN/m) (degree) pentane 18.25 37.7 hexane 20.440.1 octane 21.8 52.8 decane 23.9 59.6 dodecane 25.4 66.3 tetradecane26.7 66.5 hexadecane 27.6 73.0

cos θ of the contact angles were plotted against the surface tensions ofthe respective liquids and, by linear approximation, an approximateexpression y=−0.0546x+1.8173 (R2=0.9641, y: cos θ, x: surface tension ofliquid) was obtained. In this approximate expression, the surfacetension where cos θ became 1 was derived as a critical surface tension(γc). This γc was 14.96 mN/m.

The contact angle was measured by the following method.

1 μl of the foregoing liquid was dropped onto the surface of themagnetic disk 10 and a contact angle was measured after 10 seconds fromdropping. Measurement was carried out twice and the mean value thereofwas derived as a contact angle.

(2) Fly Stiction Test

100 magnetic disks 10 were manufactured and a whole-surface glideinspection was performed with respect to these 100 magnetic disks by theuse of a magnetic head having a flying height of 10 nm. Upon occurrenceof a fly stiction trouble, glide signals monitored by a PZT sensor(piezoelectric element) disposed in the magnetic head abruptly divergeat all tracks of the magnetic disk. Accordingly, it is possible to judgethe occurrence thereof through observation by the use of anoscilloscope. When the fly stiction occurs, the passing rate of theinspection decreases sharply. Therefore, the tendency of occurrence offly stiction can be seen based on the inspection passing rate.

As the passing rate (yield) of the fly stiction test increases, itbecomes more desirable because the cost decreases. On the other hand, ifit is 90% or more, no issue is raised. When the passing rate of the flystiction test is 80%, although there is an increase in cost, it iswithin an allowable range. The results of the fly stiction test areshown in Table 1.

(3) LUL Durability Test

An LUL durability test was performed using a 2.5-inch HDD that rotatesat 5400 rpm and a magnetic head having a flying height of 10 nm. An NPAB(negative pressure type) slider was used as a slider of the magnetichead and a GMR element was used as a reproduction element. The magneticdisk 10 was mounted in this HDD and the LUL operation was continuouslycarried out by the magnetic head. By measuring the number of LUL timesthe HDD endured without failure, the LUL durability was evaluated. Theresults of the LUL durability test are shown in Table 1.

Comparative Example 1

In the manufacture of a magnetic disk of Example 1, the processing wasperformed like in Example 1 except that the surface treatment by HFE(hydrofluoroether) was not carried out. The results are shown in Table1.

Examples 2 to 4

In the manufacture of a magnetic disk of Example 1, the processing wasperformed like in Example 1 except that the treatment time by HFE(hydrofluoroether) was changed as shown in Table 1. The results areshown in Table 1. TABLE 1 Comparative Example 1 Example 1 Example 2Example 3 Example 4 Hydro- yes no yes yes yes fluoro- (vapor (vapor(vapor (vapor ether deposition) deposition) deposition) deposition)Treatment Treatment 60 seconds — 10 20 30 Time seconds seconds secondsγSd (mN/m) 15.02 17.66 17.47 16.03 15.87 γSp (mN/m) 0.02 2.23 1.14 0.550.49 γSh (mN/m) 4.07 9.52 2.85 2.46 2.25 γS (mN/m) 19.11 29.41 21.4519.04 18.61 γSLW (mN/m) 15.02 17.66 17.47 16.03 15.87 γS⁻ (mN/m) 5.0710.45 4.24 3.85 3.42 γS⁺ (mN/m) 0.88 2.85 0.92 0.64 0.66 γS (mN/m) 19.2528.57 21.42 19.18 18.88 Critical Surface 14.96 17.12 16.93 15.55 15.42Tension [γc] (mN/m) Fly Stiction Test 98% 70% 81% 88% 93% (Passing Rate)LUL (Load Unload) durable failed at durable durable durable DurabilityTest 1,000,000 600,000 1,000,000 1,000,000 1,000,000 (Number of Durabletimes or times times or times or times or Times) more more more more

The magnetic disk of this invention can prevent a fly stiction trouble,a corrosion failure, and so on to suppress a malfunction and is thusexcellent in safety even when a magnetic head performs a flyingoperation at a flying height of, for example, 10 nm or less, and issuitably used particularly in an HDD of the LUL system.

Although this invention has been described above in detail withreference to the examples, it is needless to say that various changescan be made by a person skilled in the art without departing from thescope of claims.

1. A magnetic disk having a magnetic layer, a protection layer, and alubrication layer formed on a substrate in this order, wherein: asurface free energy γS of a surface of the magnetic disk derived by anextended Fowkes equation is greater than 0 and no greater than 24 mN/m,γSd (dispersion force component of surface free energy) forming thesurface free energy γS is greater than 0 and no greater than 17 mN/m,γSp (dipole component of surface free energy) forming the surface freeenergy γS is greater than 0 and no greater than 1 mN/m, and γSh(hydrogen bonding force component of surface free energy) forming thesurface free energy γS is greater than 0 and no greater than 6 mN/m. 2.A magnetic disk having a magnetic layer, a protection layer, and alubrication layer formed on a substrate in this order, wherein: asurface free energy γS of a surface of the magnetic disk derived by aVan-Oss equation is greater than 0 and no greater than 22 mN/m, γSLWforming the surface free energy γS is greater than 0 and no greater than17 mN/m, γS⁻ forming the surface free energy γS is greater than 0 and nogreater than 6 mN/m, and γS⁺ forming the surface free energy γS isgreater than 0 and no greater than 1 mN/m.
 3. A magnetic disk having amagnetic layer, a protection layer, and a lubrication layer formed on asubstrate in this order, wherein: a critical surface tension γc of asurface of the magnetic disk derived by a Zisman equation is greaterthan 0 and no greater than 17 mN/m.
 4. A magnetic disk according toclaim 1 or 2, wherein: a critical surface tension γc of the surface ofthe magnetic disk derived by a Zisman equation is greater than 0 and nogreater than 17 mN/m.
 5. A manufacturing method of a magnetic diskhaving a magnetic layer, a protection layer, and a lubrication layerformed on a substrate in this order, comprising: treating a surface ofsaid magnetic disk by the use of a composition containinghydrofluoroether after formation of said lubrication layer.
 6. Amanufacturing method of a magnetic disk according to claim 5, wherein:after forming said lubrication layer on a surface of said protectionlayer, said disk is heated in a clean room before and/or after thetreatment by the use of the composition containing hydrofluoroether. 7.A manufacturing method of a magnetic disk according to claim 5 or 6,wherein: said hydrofluoroether has a molecular weight of 150 to
 400. 8.A manufacturing method of a magnetic disk according to claim 5 or 6,wherein: said lubrication layer is formed by a film of aperfluoropolyether compound having polar groups at ends.
 9. Amanufacturing method of a magnetic disk according to claim 5 or 6,wherein: said protection layer is an amorphous carbon protection layerformed by a plasma CVD method.